When a cold engine is started, its lubricating oil is noticeably more viscous than when the engine is fully warmed. Consequently, the more viscous oil and may not flow as freely through oil passageways in the engine and in associated components such as a turbocharger, as when the engine is fully warmed. A result is reduced oil flow to moving parts such as bearings.
A single-stage wastegate-type turbocharger comprises a wastegate that when open, shunts flow of engine exhaust gas around an associated turbine. When the wastegate is maximally open, exhaust gas energy traveling through the turbine is minimized, thus minimizing the speed at which the turbine wheel spins. A shaft couples the turbine wheel to the compressor wheel of a compressor in the engine intake system. Consequently, when the wastegate is maximally open, the ability of the compressor to create charge air for the engine is limited. The net effect of this is to limit engine torque.
A two-stage wastegate-type turbocharger comprises a high-pressure turbine and a low-pressure turbine in series in the exhaust system. They operate a high-pressure compressor and a low-pressure compressor respectively in the intake system. A respective wastegate shunts each turbine, and it functions in the same way as described above for the single-stage turbocharger.
It is known to provide a time delay that keeps a wastegate open at engine starting and initial miming so that a maximum quantity of exhaust gas by-passes an associated turbine, minimizing the exhaust energy traveling through the associated turbine and thus minimizing the speed that the turbine wheel spins. The delay is based on an assumption that the engine is cold when started in order to limit shaft speed during a time that oil flow to the shaft bearings may be reduced due to the higher viscosity of cold oil.
It is also known to impose the time delay function only when engine oil temperature is below some limit.